1. Field of the Invention
The present invention relates to reducing power consumption in electronic devices that use batteries.
2. Background Art
A battery is a device that provides electrical energy used to power an electrical device. A battery typically includes one or more electrochemical cells that store chemical energy, which is converted to electrical energy output by the battery to provide power. Batteries are used in a multitude of electrical devices, such as electrical devices that are mobile, small, and/or unable to be constantly connected to another power source such as an AC (alternating current) power source. Batteries may also be used in electrical devices as a backup power source, to provide power when a primary power source is lost.
A rechargeable battery is a type of battery that is becoming increasingly popular. Charge in a rechargeable battery can be restored by the application of electrical energy. Rechargeable batteries based on lithium, such as lithium ion and lithium polymer batteries, are becoming increasingly widespread. A typical charging cycle for a lithium rechargeable battery includes a first charge phase, where a constant current is used to charge the battery (while battery voltage increases), and a second charge phase, where a constant voltage is applied to the battery to finish charging the battery (while the charge current decreases).
A typical lithium battery is typically shipped pre-charged to approximately 50% of its total available charge capacity to enable the battery to function properly upon receipt by a user. However, lithium batteries suffer from self-discharge when not in use. Such self-discharge may occur when a lithium battery is sitting unused on a shelf (e.g., prior to sale of the battery), such that it gradually loses energy due to undesired internal chemical actions. Thus, a lithium battery will slowly discharge prior to sale, which can be for a long time (e.g., 1-2 years). If a lithium battery discharges an amount such that a voltage of the lithium battery drops below a certain voltage value, irreversible long term damage (charge capacity loss) to the lithium battery may occur, as well as safety risks increasing.
One way to increase the shelf-lifetime of a lithium battery is to charge the lithium battery to a higher value (e.g., to 90% of its capacity) so that it takes longer to discharge. However, this is not an ideal solution because the increased amount of charging increases manufacturing costs and is not ideal for battery health. Typically, the maximum amount of charge that a lithium battery can maintain decreases with age. The higher the amount of charge stored in a lithium battery, the more rapidly that this decrease in maximum charge capacity occurs. As such, this aging effect and rate of cell self-discharge for a lithium battery increases as the amount of charge stored in the lithium battery is increased. As such, electronic devices with small lithium batteries (e.g., 50 mAH) are not able to achieve desirable store shelf-life according to conventional techniques, such as by increasing the amount of stored charge.
As such, it may be desired for lithium batteries in manufactured electronic devices to be placed in a mode that reduces battery self discharge without reducing the useful lifespan of the battery. However before the manufacturing process is complete, electronic devices typically are tested in a final form (e.g., according to an electrical verification process). Thus, the battery typically must be functional to power the electronic device during testing, and will need to be transitioned to the power down mode after such testing is complete. An external debug interface (e.g., a wired interface to a UART (universal asynchronous receiver/transmitter)) and/or a button press combination (by a human operator) may be used to test an electronic device and to enable the battery to be caused to enter the power down mode after testing, for example. However, the presence of such physical interfaces and the need for manual interaction for device testing and shut down are undesirable.